WO2024143706A1

WO2024143706A1 - Method for driving electro-absorption modulator operating in burst mode

Info

Publication number: WO2024143706A1
Application number: PCT/KR2023/003638
Authority: WO
Inventors: 김정수
Original assignee: 주식회사 포벨
Priority date: 2022-12-27
Filing date: 2023-03-20
Publication date: 2024-07-04

Abstract

The present invention relates to a method for driving an electro-absorption modulator operating in a burst mode. In the provided method for driving an electro-absorption modulator operating in a burst mode, according to the present invention, the magnitude of DC voltage and AC voltage for driving the electro-absorption modulator is functionally reduced according to the elapse of time from the time at which laser light is injected into the electro-absorption modulator so that light absorption begins, and thus offset light absorption characteristics due to changes in temperature of the electro-absorption modulator area can be offset.

Description

Driving method of a field absorption modulator operating in burst mode

The present invention relates to a method of driving a modulator, and particularly to a method of driving an electric field absorption modulator that operates in burst mode.

Currently, light emitting devices used in high-speed optical communications are used in the form of modulator-integrated semiconductor lasers. In a modulator integrated semiconductor laser (Electro-Absorption Modulator Integrated DFB-LD: EML), the DFB-LD (Distributed Feed Back-Laser Diode) operates continuously and an optical modulator is mounted on the optical output part of the DFB-LD to operate the optical modulator. Create a modulation signal using .

The operating characteristics of this optical modulator are shown in Figure 1.

The absorption spectrum curve according to the wavelength when no reverse voltage is applied to the modulator moves to a longer wavelength as the absorption spectrum curve decreases when a reverse voltage is applied to the modulator. In Figure 1, the modulator has a low absorption rate of B1 when the electric field E = 0 V/cm for the B-wavelength light source entering the modulator, but when a voltage of 5.4E4 V/cm is applied to the modulator, the absorption spectrum of the modulator changes. Therefore, B2 has a high absorption rate, and the intensity of light passing through the modulator varies from the difference in absorption rate between B1 and B2. A device in which the intensity of light passing through such a modulator is determined by the magnitude of the reverse voltage applied to the modulator is called an electro absorption modulator.

Depending on the band gap of the modulator itself and the wavelength of the incoming light, modulator operation may occur at wavelengths C in FIG. 1 and D in FIG. 1 even for the same modulator. Therefore, a certain, well-controlled relationship is required between the band gap of the modulator itself and the incoming wavelength. Figure 2 is a case where the absorption spectrum of the modulator itself was measured at a temperature of 300K, and is a diagram showing the change in the absorption spectrum when a voltage is applied to the modulator maintained at 300K. However, when all substances absorb energy, their temperature changes, and this varies depending on the amount of energy absorbed and the heat dissipation of the substance. The band gap of the semiconductor material used as an absorbing material in the modulator changes depending on the temperature, and the absorption spectrum shifts according to the electric field from this changed band gap wavelength. This is shown in Figure 2.

In Figure 3, the blue absorption spectrum curve is the absorption spectrum curve according to temperature when no voltage is applied, and the yellowish yellow color is the absorption spectrum curve moving according to voltage at each temperature. Therefore, in order to obtain the intensity of light passing through the modulator and an accurate extinction ratio (ratio of the signal intensity of the “1” signal and the “0” signal), the temperature and applied voltage of the modulator must be accurately controlled. That is, in FIG. 2, the extinction ratio when zero voltage (E=0 V/cm) and E=5.4E4 V/cm are applied to the input light wavelength of “B” at a temperature of 300K, and the temperature of the modulator at a temperature of 310K. When zero voltage (E=0 V/cm) and E=5.4E4 V/cm are applied to the same input light wavelength, the extinction ratio changes and the average light output also changes.

The optical signal from which the “1” and “0” signals are generated in the modulator is transmitted through the optical fiber, and the optical receiving end uses the statistical power and signal extinction ratio of the “1” and “0” signals to produce “1” and “0” signals. Extract . Therefore, when the modulator is driven using the same voltage from the moment of operation, the temperature of the modulator changes over time, and the average power and extinction ratio of the signal generated from the modulator change accordingly, which changes the signal at the receiving end. It acts as an error.

The conventional electric field absorption modulator operates in CW (continuous wave, continuous operation), and after an initial period of time, the temperature of the modulator is stabilized and no additional temperature change in the modulator area occurs, whereas in burst mode as shown in FIG. 4, the temperature of the modulator is stabilized. In a modulator operating in (Burst Mode), the operation changes between the time when light is not applied to the modulator and the time when light is applied to the modulator to form a modulation signal, so the temperature of the light absorption area of the modulator is continuously maintained according to the burst operation. changes, and as a result, signal transmission using a burst mode modulator becomes difficult.

[Prior art literature]

[Patent Document]

(Patent Document 1) Republic of Korea Patent Publication No. 10-0362060 (registered on November 11, 2002)

The present invention was proposed to solve the problems occurring in the conventional electric field absorption modulator operating in burst mode. The purpose of the present invention is to provide a modulator according to the operation of a light source in an electric field absorption modulator operating in burst mode. There is a change in the intensity of the light source coming into the modulator, and as a result, the temperature of the modulator changes depending on the intensity of the light absorbed by the modulator. In the modulator driving conditions that do not take into account the temperature change of the modulator, the light passing through the modulator changes over time. The goal is to provide a method of driving a field absorption modulator that can solve the problem of difficulty in distinguishing signals at the optical receiving end due to changes in average intensity and extinction ratio.

A method of driving an electric field absorption modulator according to the present invention to achieve the above object is a method of driving an electric field absorption modulator operating in burst mode, wherein a DC voltage for driving the electric field absorption modulator The magnitude of the AC voltage decreases as a function of time from the point when laser light is injected into the field absorption modulator and light absorption begins.

At this time, the function by which the magnitude of the DC voltage and AC voltage decreases over time is predetermined and recorded in the memory.

Meanwhile, it is preferable that a laser diode as a light source is manufactured and operated integrally with the optical input part of the field absorption modulator.

Additionally, an optical amplifier may be manufactured and operated integrally with the optical output portion of the field absorption modulator.

The electric field absorption modulator may be a ridge type modulator having a deep ridge structure.

Meanwhile, the “1” signal and “0” signal of the field absorption modulator produce a “0” signal with a DC voltage +1/2 modulation voltage, and “1” with a DC voltage - 1/2 modulation voltage. ” It is desirable to generate a signal or apply a modulation voltage in an equivalent process, and that the reduction function of the DC voltage and the modulation voltage after the field absorption modulator operates decreases exponentially with time.

In addition, the “1” signal and “0” signal of the field absorption modulator produce a “0” signal with a DC voltage +1/2 modulation voltage, and “1” with a DC voltage - 1/2 modulation voltage. ” Generates a signal or applies a modulation voltage in an equivalent process, and the reduction function of the DC voltage and modulation voltage after the field absorption modulator is operated is a function that has the characteristic of offsetting the temperature increase of the field absorption modulator over time. You can.

The laser diode may have a reverse mesa structure with a heater mounted on the top.

In addition, the electric field absorption modulator preferably has a reverse mesa structure with a heater mounted on the top.

Meanwhile, the heater mounted on top of the field absorption modulator is driven to offset temperature changes due to light absorption of the modulator operating in burst mode.

Additionally, the heater mounted on the top of the field absorption modulator may be mounted separated by an insulating film on the top electrode of the modulator.

The heater mounted on top of the field absorption modulator may be connected to an external electrode in the form of an air bridge.

According to the method of driving a field absorption modulator operating in burst mode of the present invention, the temperature change due to light absorption of the modulator operating in burst mode and the change in light output and extinction ratio due to temperature change are suppressed to improve signal discrimination at the optical receiving end. It has a facilitating effect.

1 is a graph of the change in absorption spectrum according to the applied voltage of the electric field absorption modulator;

Figure 2 shows the operating principle according to the driving voltage of the modulator,

Figure 3 is an example showing that the modulation signal extinction ratio changes depending on the temperature of the modulator when driving the modulator with a constant driving voltage regardless of the modulator temperature;

4 is an example of burst mode operation of the modulator;

Figure 5 is an example of the optical output characteristics of a modulator modulated by changing the driving voltage of the modulator to offset the temperature change in the absorption region of the modulator according to the present invention;

Figure 6 is an example of temperature change according to light absorption time in the modulator area operating in burst mode according to the present invention;

Figure 7 is another example of the temperature change in the modulator area due to light absorption from the start of operation of the modulator according to the present invention.

Figure 8 is an example of a reverse mesa ridge structured light source equipped with a heater according to the present invention;

Figure 9 shows an example of a modulator with a reverse mesa ridge structure having a deep ridge structure on which a heater according to the present invention is mounted.

10 is an example of a heater driving method in a modulator equipped with a heater according to the present invention;

Figure 11 shows an example of the temperature change of the modulator when the burst on starts in the modulator, light absorption occurs, and the heater is stopped at the same time so that heat generation by the heater disappears according to the present invention.

Hereinafter, non-limiting preferred embodiments of the present invention will be described in detail with reference to the attached drawings.

Figure 5 shows an example of the optical output characteristics of a modulator modulated by changing the driving voltage of the modulator to offset temperature changes in the absorption region of the modulator according to the present invention.

Figure 5 shows the extinction ratio under the driving voltage conditions of E=0 v/cm and E=5.4 V/cm, shown in blue at a temperature of 300K. The solid blue line is the absorption spectrum at 300K and E=0 v/cm, in which case almost no absorption of the “B” wavelength occurs. The blue dotted line shows absorption when the electric field is 5,4E4 V/cm, from which the initial extinction ratio is determined. However, when this modulator begins to absorb light and the temperature of the modulator increases to 310K, if the same electric fields of 0 V/cm and 5.4E4 V/cm are applied, the extinction ratio becomes very large. Therefore, if the electric field is kept the same, the temperature of the modulator area changes over time and the extinction ratio changes accordingly.

In Figure 5, when the temperature of the modulator rises to 310K and the electric field of the modulator is changed to 0 V/cm and 2.1E4 V/cm, the extinction ratio obtained at 300K, 0 V/cm and 5.4E4 V/cm It can be seen that a similar extinction ratio can be obtained.

As shown in Figure 5, the change in extinction ratio due to the temperature change due to the start of the burst mode and the change in the absorption curve according to the temperature change of the modulator is achieved by changing the driving voltage in a way to minimize the change in the extinction ratio. The change in extinction ratio at the beginning and end can be minimized.

Although the present invention is explained using an electric field, since the length of the electric field direction of the actually manufactured modulator is fixed, a change in the electric field can be implemented as a change in the voltage applied to the modulator. Typically, the voltage of the modulator is given as the sum of the DC voltage and the AC modulation voltage corresponding to the “1” and “0” signals. Therefore, the change in extinction ratio according to the change in modulator temperature is the DC voltage and modulation according to the change in modulator temperature. It can be offset by a change in voltage.

“1” signal voltage = DC voltage - 1/2 AC modulation voltage

“0” signal voltage = DC voltage + 1/2 AC modulation voltage

However, the modulation voltage does not necessarily have to be implemented in this way, and various methods are possible for applying voltage according to the “1” and “0” signals.

Basically, a change in the temperature of the modulator leads to a change in the absorption spectrum, and accordingly, in order to obtain a change in the modulation voltage to offset the change in the modulation characteristics of the modulator, the temperature change of the modulator according to the operating time of the modulator must first be measured. The change in temperature from the start of operation of the modulator varies depending on the structure of the modulator and the intensity of light entering the modulator. Figures 6 and 7 show an example of the change in temperature from the start of operation of the modulator.

In Figures 6 and 7, the temperature change of the modulator can be simulated using an exponential function and a high order term of the fourth square term. Since the wavelength shift of the modulator changes almost linearly with temperature, as in the example shown in Figures 6 and 7, the temperature change of the modulator is first simulated, the resulting change in the modulator spectrum is inferred, and then the characteristics such as the extinction ratio of the modulator are changed accordingly. The DC voltage and AC modulation voltage to cancel can be obtained as a function of time. There may be various changes in FIGS. 6 and 7, but the important point is that the changed DC voltage and AC modulation voltage are applied to the modulator to offset temperature changes over time and changes in modulator characteristics at each time.

It is possible to replace the changes in the DC voltage and AC modulation voltage of the modulator with a function from the start of the burst signal and adjust the actual modulation voltage of the modulator from this function. Alternatively, a look-up table according to time can be stored in memory, A method of driving the modulator can be used by reading the DC voltage and AC modulation voltage for driving the modulator from this memory.

In order to manufacture a modulator that operates in burst mode, a light source that operates in burst mode is required, and the modulator and light source are typically manufactured as an integrated unit. These light sources consist of DFB-LD or DBR-LD (Distributed Bragg Reflector-Laser Diode), and the light source wavelength itself changes depending on the burst mode operation of these light sources. Therefore, changes in the light source wavelength itself must be suppressed, and it is desirable to embed an electric heater on one side or top of the DFB-LD. In order to facilitate the installation of such a heater, the light source structure of the DFB-LD has an inverse mesa ridge structure. A structure (reverse mesa - ridge structure) is appropriate.

Figure 8 shows an example of a reverse mesa ridge structure light source equipped with a heater according to an embodiment of the present invention. The current that operates the light source is injected into the DFB-LD electrode (p-type electrode) of Figure 8, and the heater is It is placed on top of the DFB-LD electrode and is driven complementary to the current that operates the light source to offset temperature changes in the laser diode area due to the burst mode operation of the laser diode. In other words, the heater is mounted on the top of the DFB-LD light source, and the structure of the DBB-LD light source has a reverse mesa ridge structure to make it easy for the heater to be mounted on the top. Reverse mesa ridge refers to a structure where the upper part of a clad layer composed of p-InP/P-InGaAs layers is wider than the lower part. In FIG. 8, since the electrode for driving the light source is disposed on the upper part of the mesa, it is preferable that the heater is separated by an insulating film and overlapped on the upper electrode for driving the light source.

The characteristic to be improved in the present invention is that the period in which the modulator absorbs light and transmits modulation appears in the form of a burst, and in the period other than this burst on time, there is no light absorption in the modulator, so the modulator cools down and bursts. During the burst on period, light absorption begins and the temperature of the modulator rises, and thus the light absorption characteristics also change over time. Therefore, the above-described method maintains the light absorption characteristics of the modulator constant by using a change in the driving voltage of the modulator to cancel out the temperature change of the modulator, while keeping the light absorption characteristics of the light modulator constant after burst on. A method of maintaining the condition is to install a heater on the modulator.

Figure 9 shows an example of a modulator with a reverse mesa ridge structure and a deep ridge structure on which a heater is mounted according to an embodiment of the present invention.

In Figure 9, two grooves are formed on both sides of the inverse mesa ridge structure modulator on the substrate, and these grooves form a deep ridge structure, and this deep ridge structure allows the electric field of the modulator to be concentrated in the active area. Improve the characteristics of the modulator. In Figure 9, a heater is installed in the modulator area and the heater is kept turned off while the modulator performs the role of light absorption/light modulation/light transmission, and when the modulator cools down because it does not play the role of light absorption/light modulation/light transmission. It shows how to prevent temperature changes due to light absorption of the modulator and changes in light modulation characteristics due to temperature changes by operating a heater to prevent cooling and maintaining the modulator at a constant temperature regardless of whether the modulator is operating. .

In FIG. 9, the heater is mounted on the top of the modulator, and the structure of the modulator preferably has an inverse mesa ridge structure so that the heater can be easily mounted on the top. Inverse mesa ridge refers to a structure where the upper part of the clad layer composed of p-InP/P-InGaAs layer is wider than the lower part. The width of the lower part of the ridge is 1~2um so that light has a single transverse mode. Compared to the width of , the width of the upper part of the ridge can be at least 0.5um more than this. More preferably, the cross-section of the ridge appears as a (111) plane. In this case, the width of the upper part of the ridge can be 2 to 3 μm wider than the width of the lower part of the ridge, making it easier to install additional heaters.

In FIG. 9, it is desirable for the high-speed modulator to minimize capacitance, and accordingly, the heater electrode is also preferably connected to the external electrode in the form of an air bridge to minimize parasitic capacitance. This air bridge-type electrode is manufactured in a way that the part where the electrode metal connects to the pad is floating in the air. This minimizes the parasitic capacitance that occurs at the connection part of the heater pad and facilitates high-speed operation of the device. . In addition, since the electrode for driving the modulator is disposed on the upper part of the mesa, it is preferable that the heater is separated by an insulating film and overlapped on the upper electrode for driving the modulator. In FIG. 9, for simplification of explanation, an insulating film to protect InP is not shown, but a surface protective layer such as an insulating film of a general modulator structure may be added.

Figure 10 shows a heater driving method in a modulator equipped with a heater. The heater is turned off during the burst on period when light absorption/light modulation/light transmission occurs in the modulator, and during the burst-off period when light does not enter the modulator, the heater operates to absorb the light absorption power generated by the modulator. By replacing , the temperature of the modulator is kept constant regardless of burst on/off.

Figure 11 shows the temperature change of the modulator when the burst on begins in the modulator and light absorption occurs, and at the same time, the heater is stopped and heat generation by the heater disappears. The temperature by the heater, which is the curve in Figure 11 (a), shows the temperature change of the modulator over time when the heater stops generating heat when there is no light absorption, and the temperature of light absorption, which is the curve in Figure 11 (b), is It shows the temperature change of the modulator when light absorption begins in the modulator while the heater is not operating. Therefore, when these two effects occur simultaneously, it appears as the average effect of the two effects (a) and (b) in the curve in (c) of Figure 11, and burst on/off as shown in the curve in (c) of Figure 11. ) Regardless, the temperature of the modulator is maintained constant, allowing stable modulator characteristics.

As such, in the present invention, the DC voltage and the modulation voltage that drive the modulator are changed as a function of time, and the modulator is driven to offset the temperature change of the modulator according to the modulator driving time, thereby increasing the average intensity and extinction ratio of the output optical signal of the modulator. can be kept constant.

Meanwhile, the power modulation of the heater in FIG. 9 described above does not require alternating between a predetermined power and 0 power, and the purpose of the present invention can be achieved if the heater modulation power offsets the change in heat amount due to light absorption by the modulator. .

In addition, in Figures 8 and 9, a light source such as DFB-LD is explained as having an inverse mesa ridge structure, and the modulator also has an inverse mesa ridge structure, but an inverse mesa light source such as DBR-LD with a mesa structure and a heater are installed. Combinations of modulator structures are also possible.

As such, the present invention is not limited to the above-described embodiments, and various modifications may be made by those skilled in the art to which the present invention pertains within the scope of equivalency of the technical idea of the present invention and the scope of the claims described below. Of course, modifications may be made.

Claims

In a method of driving an electric field absorption modulator operating in burst mode,

The magnitude of the DC voltage and AC voltage driving the field absorption modulator decreases as a function of time from the time the laser light is injected into the field absorption modulator and light absorption begins. How to drive.
In claim 1,

A method of driving a field absorption modulator, wherein the function by which the magnitude of the DC voltage and AC voltage decreases over time is predetermined and recorded in a memory.
In claim 1,

A method of driving a field absorption modulator, characterized in that a laser diode as a light source is manufactured and operated integrally in the optical input part of the field absorption modulator.
In claim 1,

A modulator driving method characterized in that an optical amplifier is manufactured and operated integrally with the optical output portion of the field absorption modulator.
In claim 1,

A method of driving a field absorption modulator, characterized in that the field absorption modulator is a ridge type modulator having a deep ridge structure.
In claim 1,

The “1” signal and “0” signal of the field absorption modulator produce a “0” signal with a DC voltage +1/2 modulation voltage, and a “1” signal with a DC voltage - 1/2 modulation voltage. generates or applies a modulation voltage of an equivalent process,

A method of driving a field absorption modulator, wherein the reduction function of the DC voltage and modulation voltage after the field absorption modulator is operated decreases exponentially with time,
In claim 1,

The “1” signal and “0” signal of the field absorption modulator produce a “0” signal with a DC voltage +1/2 modulation voltage, and a “1” signal with a DC voltage - 1/2 modulation voltage. generates or applies a modulation voltage of an equivalent process,

A method of driving a field absorption modulator, characterized in that the reduction function of the DC voltage and the modulation voltage after the field absorption modulator is operated is a function that has the characteristic of offsetting a temperature increase of the field absorption modulator over time.
In claim 3,

A method of driving a field absorption modulator, wherein the laser diode has a reverse mesa structure with a heater mounted on the top.
In claim 1,

A method of driving a field absorption modulator, characterized in that the field absorption modulator has a reverse mesa structure with a heater mounted on the top.
In claim 9,

A method of driving a field absorption modulator, characterized in that the heater mounted on the top of the field absorption modulator is driven to offset temperature changes due to light absorption of the modulator operating in burst mode.
In claim 9,

A method of driving a field absorption modulator, characterized in that the heater mounted on the upper part of the field absorption modulator is separated by an insulating film on the upper electrode of the modulator.
In claim 9,

A method of driving a field absorption modulator, characterized in that the heater mounted on the top of the field absorption modulator is connected to an external electrode in the form of an air bridge.